Method of manufacturing sulphuric acid

ABSTRACT

A process is provided for manufacturing sulfuric acid from a gas containing sulfur dioxide and steam. The sulfur dioxide and steam containing gas is subjected to a catalytic oxidation to convert the sulfur dioxide into sulfur trioxide so as to produce a gas having a sulfur trioxide to steam mole ratio of at least 1:1 at a temperature of 400 to 600° C. This sulfur trioxide and steam containing gas is injected into an indirect heat exchanger where it is indirectly cooled with a cooling fluid so that at least 80 percent of the theoretically possible sulfuric acid is formed and condensed in this heat exchanger. The cooling fluid exiting the heat exchanger is fed into a turbine for the generation of electricity. The gas mixture exiting the heat exchanger is contacted with sulfuric acid in a separate contact vessel to form additional, concentrated sulfuric acid. The sulfuric acid formed in the heat exchanger is discharged.

This invention relates to a process of producing sulfuric acid from asteam-containing gas mixture, which contains SO₃ and H₂ O in a molarratio of at least 1:1 and comes from a catalytic SO₂ oxidation with atemperature of 300-600° C.

Many variants of the production of sulfuric acid and details an variousprocess stages, e.g. also on the catalytic SO₂ oxidation, are describedin Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol.A25, pp. 647-664. It is the object of the invention to produce sulfuricacid as inexpensively as possible. This process should allow for makinguse of the energy of the steam-containing gas mixture, which comes fromthe catalytic SO₂ oxidation.

In accordance with the invention, this is achieved in theabove-mentioned process in that the gas mixture containing SO₃ and steamis passed through a condenser, in which the gas mixture is indirectlycooled by means of a cooling fluid to a temperature of 180-240° C. atthe outlet of the condenser, where at least 50% of the theoreticallypossible sulfuric acid are formed, condensed and discharged, and thecooling fluid, which has been heated to a temperature of at least 130°C., is withdrawn from the condenser, and that an SO₃ -containing gasmixture is withdrawn from the condenser, the gas mixture is brought incontact with concentrated sulfuric acid at a temperature of 60-120° C.,and concentrated product sulfuric acid is withdrawn. The cooling fluidused in the condenser may be liquid or gaseous, and it can for instancebe cooling water or air.

When the cooling fluid supplied to the condenser is water, steam of 2.5to 10 bar is generated in the condenser, which steam is a valuableproduct. This steam can for instance be utilized for operating aturbine, which produces electrical energy. When the indirect cooling inthe condenser is effected by means of gas (e.g. air), the energy of theheated gas can also usefully be passed on outside the condenser, and forinstance preheated air can be used as combustion air.

Preferably, at least 80% of the theoretically possible sulfuric acid areformed in the condenser, which are withdrawn as condensate. Thissulfuric acid usually has a H₂ SO₄ concentration of at least 97 wt-%.

Embodiments of the process will now be explained with reference to thedrawing, wherein:

FIG. 1 shows a flow diagram of the process, and

FIG. 2 shows a condenser designed as electrostatic precipitator.

Through line 1, see FIG. 1, SO₂ -containing gas is supplied, e.g. gasfrom a sulfur combustion or another SO₂ source. Usually, the gas alsocontains steam and air. The gas in line 1 is treated in a first plant 2for the catalytic SO₂ oxidation. Plants of this type are known. At atemperature of usually 400-600° C. a gas mixture containing SO₃ andsteam leaves the plant 2 via line 3 and is first of all passed throughan indirect cooler 4, which serves the pre-cooling of the gas mixture.Usually, the gas mixture in line 3 also contains SO₂. In the cooler 4,the gas mixture is cooled to temperatures of 300° C., so that acondensation does not yet take place. The cooler 4 is expedient for therecovery of energy, but not absolutely necessary.

The steam-containing gas mixture in line 6 has a temperature in therange from 300-600° C. and a pressure of 0.8 to 2 bar. This gas mixtureis cooled in the condensers to a temperature in the range from 180-240°C., where sulfuric acid is condensed. The condenser 7 comprises an inletchamber 7a, a bundle of about horizontal tubes 7b, a supply line 7c forcooling fluid, a cooling fluid chamber 7d, and an outlet chamber 7e.Expediently, the cooling fluid is preheated in the heat exchanger 12,before it enters the condenser 7. The cooling fluid heated in thecondenser to a temperature of at least 130° C. and preferably at least150° C. is discharged via line 9. When water is supplied as coolingfluid through line 7c, steam is generated, which is discharged via line9 at a pressure of about 3-10 bar. In the process illustrated in thedrawing, the steam flows to an expansion turbine 10, which drives anelectric generator 11.

While the gas mixture containing SO₃ and steam flows through the tubes7b and is cooled in the process, there is first of all formed vaporoussulfuric acid. During the further cooling in the tubes 7b, the sulfuricacid formed will condense and flow first to the outlet chamber 7e with aH₂ SO₄ content of about 98 to 100%, and then through line 14 to acollecting tank 15. The conditions in the condenser 7 are chosen suchthat at least 50% and preferably at least 80% of the theoreticallypossible sulfuric acid, which can be formed from existing SO₃ plus H₂ O,is condensed and withdrawn.

From the outlet chamber 7e, a gas mixture containing residual SO₃ iswithdrawn through line 16, which gas mixture usually also contains O₂,SO₂ and sulfuric acid vapor. This gas mixture flows to a first contactzone 18 of the type of a Venturi scrubber, which serves as SO₃ absorber.Concentrated sulfuric acid is supplied through line 19 and sprayed intothe first contact zone 18. Discharged sulfuric acid flows through line20 into the collecting tank 15. The residual gases flow through aconnecting passage 21 into a second contact zone 22, an SO₃ absorptioncolumn with a packed bed 23, and a supply line 24 for concentratedsulfuric acid. In the zone 22 the gases flow upwards in contact withsulfuric acid trickling downwards. From the lower portion of the secondcontact zone 22 the concentrated sulfuric acid flows through line 26likewise into the collecting tank 15. From the tank 15 concentratedsulfuric acid is withdrawn via line 27, is passed through an indirectcooler 28, where the temperature of the cooled sulfuric acid is broughtinto the range from 60 to 120° C. and usually from 70 to 100° C. Part ofthe cooled sulfuric acid is discharged as product acid through line 30,whereas the major part circulates through line 24 to the second contactzone 22 and through line 19 to the first contact zone 18. Usually, theconcentration of the sulfuric acid in line 27 lies in the range from97-99.5 wt-% H₂ SO₄. In the case of relatively small gas quantities inline 16, the first contact zone 18 may be omitted, and the gas may bepassed directly to the lower portion of the second contact zone 22,which is, however, not illustrated in the drawing.

The residual gas flowing upwards in the second contact zone 22 reaches amist eliminator 34 and then leaves the contact zone via line 35. Thisgas is virtually free from SO₃, steam and sulfuric acid vapor, and aboveall still contains SO₂ and O₂. The residual gas is passed through a heatexchanger 36, where it is heated to the inlet temperature of 380-420°C., which in the succeeding second plant 37 is necessary for thecatalytic SO₂ oxidation. For the further production of sulfuric acid,SO₃ -containing gas formed is supplied through line 38 to a finalabsorption stage 39, which operates for instance in a manner known perse, which is not represented in detail in the drawing: The gas in line38 is indirectly cooled to 140-200° C., and in an SO₃ absorption columncontaining a packed bed is trickled with concentrated sulfuric acid(98-99.5 wt-% H₂ SO₄). The plant 37 and the stage 39 are, however, notnecessary in any case, and above all serve the observance ofenvironmental regulations. Instead of the plant 37 and the stage 39, gasscrubbing methods known per se can also be used.

The variant of the condenser 7 schematically represented in FIG. 2 isdesigned as wet-type electrostatic precipitator. In the inlet chamber7a, the steam-containing gas mixture supplied via line 6 is sprayed withsulfuric acid from line 40 and the distributor 41, in order to increasethe moisture of the gas, which is important for the efficiency of theelectrostatic precipitator. The electrostatic precipitator comprisesnumerous precipitating electrodes 42, which are for instance designed astubes, through which flows the gas mixture. The counter-electrodes inthe form of ionizer wires 43, which are associated to the precipitatingelectrodes, are indicated in dotted lines. The cooling fluid, whichflows around the precipitating electrodes, flows in through line 7C andout through line 9. By means of the electrostatic precipitator, mistdroplets are removed from the gas mixture very efficiently. Gas andcondensed sulfuric acid flow through the passage 44 to the outletchamber 7e, where the gas is discharged via line 16 and subjected to afurther treatment as it is explained in conjunction with FIG. 1. Thesulfuric acid is discharged via line 14, see also FIG. 1, and in partrecirculated via line 40.

EXAMPLES

There is used a plant in accordance with FIG. 1 of the drawing, wherewater is supplied to the condenser 7 as coolant, and saturated steam iswithdrawn via line 9. In any case, the water is boiler feed watersupplied at a temperature of 105° C., which is preheated in the heatexchanger 12 to an elevated temperature. The plant components 10, 11, 37and 39 are omitted. The data have been calculated in part.

Example 1

For the production of 110 daily tons H₂ SO₄ an SO₂ gas of metallurgicalorigin is used, which is charged into a conventional plant 2 for thecatalysis of SO₂. Gas quantities, gas components and temperatures invarious lines are indicated in

                  TABLE I                                                         ______________________________________                                        Line           3       6         16    35                                     ______________________________________                                        Gas quantity (Nm.sup.3 /h)                                                                   11,517  11,151    9,726 9,626                                  SO.sub.3 (kg/h)                                                                               3,480   2,175    179   <0.1                                   H.sub.2 O (kg/h)                                                                               792     498     --    --                                     H.sub.2 SO.sub.4 (kg/h)                                                                      --       1,599    517   <0.2                                   Temperature (° C.)                                                                      465     380     200   80                                     ______________________________________                                    

The cooling water in an amount of 3,500 kg/h was first of all preheatedto 140° C. through an indirect heat exchange with the condensate in line14, before it entered the condenser 7. Via line 9, 3,356 kg/h saturatedsteam of 7 bar were withdrawn.

Example 2

For the production of 500 daily tons H₂ SO₄ SO₂ -containing calcinationgases from the calcination of zinc concentrate are used. The contactzone 18 is omitted.

                  TABLE II                                                        ______________________________________                                        Line           3       6         16    35                                     ______________________________________                                        Gas quantity (Nm.sup.3 /h)                                                                   59,859  56,886    52,233                                                                              51,459                                 SO.sub.3 (kg/h)                                                                              15,400   4,774     1,024                                                                              --                                     H.sub.2 O (kg/h)                                                                              3,363    972     --    --                                     H.sub.2 SO.sub.4 (kg/h)                                                                      --      13,017     2,220                                                                              <0.6                                   Temperature (° C.)                                                                      490     320       205 80                                     ______________________________________                                    

The gas in line 35 is passed through the heat exchanger 4. The boilerfeed water entering the condenser 7 in an amount of 12,130 kg/h has beenpreheated to 150° C. 10,178 kg/h saturated steam of 7 bar are generatedand withdrawn via line 9.

Example 3

There is processed an acid gas from the desulfurization plant of an oilrefinery. Together with the gas liquid sulfur is burnt, in order toenrich the gas with SO₃. The cooler 4 serves the superheating ofsaturated steam of 42 bar to 360° C.

                  TABLE III                                                       ______________________________________                                        Line           3       6         16    35                                     ______________________________________                                        Gas quantity (Nm.sup.3 /h)                                                                   13,206  13,138    12,349                                                                              12,277                                 SO.sub.3 (kg/h)                                                                               1,528   1,282       96 <0.5                                   H.sub.2 O (kg/h)                                                                               332     277     --    --                                     H.sub.2 SO.sub.4 (kg/h)                                                                        439     740       198 <0.1                                   Temperature (° C.)                                                                      420     390       185 75                                     ______________________________________                                    

From 2.675 kg/h water, which enters the condenser 7 after having beenpreheated to 123° C., 2,620 kg/h saturated steam of 5 bar are generated.In the cooler 36 2,600 kg/h saturated steam of 42 bar are superheatedfrom 253° C. to 360° C.

Example 4

For the production of 1,500 daily tons H₂ SO₄ an SO₂ gas from a gold orecalcination plant are used.

                  TABLE IV                                                        ______________________________________                                        Line           3       6         16    35                                     ______________________________________                                        Gas quantity (Nm.sup.3 /h)                                                                   108,277 103,268   83,720                                                                              82,234                                 SO.sub.3 (kg/h)                                                                               48,395  30,490    3,980                                                                              <2.0                                   H.sub.2 O (kg/h)                                                                              10,267  6,238    --    --                                     O.sub.2 (kg/h)  12,524  12,524   12,524                                                                              12,524                                 H.sub.2 SO.sub.4 (kg/h)                                                                      --       21,934    1,757                                                                              <1.2                                   Temperature (° C.)                                                                      463     370       190 85                                     ______________________________________                                    

From 40,000 kg/h boiler feed water, preheated to 141 C., an amount of38,700 kg/h saturated steam of 7 bar is generated, which is withdrawnvia line 9.

What is claimed is:
 1. A process for producing sulfuric acid from a gascontaining sulfur dioxide and steam which comprises the steps of:(a)catalytically oxidizing the sulfur dioxide in said gas to sulfurtrioxide to obtain a gas mixture which contains SO₃ and H₂ O in a molarratio of at least 1:1 and is at a temperature of 400 to 600° C.; (b)passing the gas mixture containing SO₃ and H₂ O through a condenser inwhich the gas mixture is indirectly cooled by means of a cooling fluidso that at least 80% of the theoretically possible sulfuric acid isformed and condensed; (c) withdrawing from said condenser condensedsulfuric acid at a concentration of at least 97 weight % at atemperature of 180 to 240° C.; (d) withdrawing the cooling fluid fromsaid condenser; (e) withdrawing an SO₃ -containing residual gas mixturefrom the condenser; (f) directly charging the SO₃ -containing residualgas mixture and concentrated sulfuric acid into a contact vessel at atemperature of 60 to 120° C. to form additional concentrated sulfuricacid from the SO₃ -containing residual gas; and (g) withdrawing theconcentrated sulfuric acid obtained during step (f).
 2. The processdefined in claim 1 wherein the cooling fluid supplied to the condenseris water, and steam of 2.5 to 10 bar is generated in the condenser. 3.The process defined in claim 1 wherein following step (a) the gasmixture containing SO₃ and steam is subjected to a pre-cooling withoutformation of a condensate, before the gas mixture is introduced into thecondenser.
 4. The process defined in claim 1 wherein the condenser is anelectrostatic precipitator with cooled electrodes.
 5. The processdefined in claim 1 wherein following step (d) the cooling fluidwithdrawn from the condenser is fed into a turbine for generatingelectricity.